Apparatus, such as mobile phones and wireless LAN systems, includes a PLL circuit so that the oscillator section can produce a high frequency signal. The PLL circuit and associated circuitry consume a lot of power. To reduce power consumption in such apparatus, the PLL circuit is activated only when necessary, for example, in transmission and reception. That is, the PLL circuit is frequently turned on/off. A conventional, popular approach to reduce power consumption in mobile phones, wireless LAN systems, and like apparatus is to power off the PLL circuit unless it is being used in transmission or reception (“standby period”).
The PLL circuit however has a problem that it takes time to produce a stable frequency after the power supply is turned on. This is due to the structure of the PLL circuit. The PLL circuit contains a voltage-controlled oscillator (VCO) with a feedback loop. The oscillation control voltage fed back to the VCO is altered in such a manner to eliminate a phase difference between a reference signal and an oscillation signal, hence achieving a target oscillation frequency.
FIG. 9 shows how power consumption in the transceiver block of a conventional wireless LAN system changes with time when receiving a data packet. In the figure, the numbers, 901 to 904, indicate the power consumption levels of the transceiver block. 901 indicates a power consumption level when receiving a data packet; 902 when transmitting an ACK signal as a response to the reception of the data packet; and 903 and 904 when standing by with the power supply being kept on and turned off respectively. The transceiver block can save power consumption by an equivalent to the hatched area in the figure if the power supply is turned off during the standby period (as compared with the case where the power supply remains turned on).
Assuming a data transfer rate of 512 kbps and a packet cycle of 20 ms under the IEEE 802.11b standards, for example, the hatched area in FIG. 9 (i.e. standby period) lasts for about 18.7 ms, which means that the standby period accounts for about 94% of the entire packet cycle. Therefore, reducing power consumption by turning off the power supply to the transceiver block during the standby period has a great impact on reducing power consumption in the wireless LAN system. In addition, given a data transfer rate of 1.4 Mbps and a packet cycle of 8 ms, the standby period is about 6.5 ms, which means that the standby period accounts for about 81% of the entire packet cycle. As would be understood from these figures, the standby period decreases with an increase in the data transfer rate. Nevertheless, the standby period still accounts for a large portion of the entire packet cycle. Thus, turning off the power supply during the standby period is an effective way to reduce power consumption in the wireless LAN system.
To turn on the power supply to the wireless LAN system in the off state, the PLL circuit's frequency must become stable first. Put differently, the PLL circuit's frequency must be stable before the wireless LAN system starts receiving a signal. The pull-in time, required for the PLL circuit to produce a stable frequency, is typical about 1 to 10 ms. Therefore, the power supply to the wireless LAN system should be turned on about 1 to 10 ms before the wireless LAN system starts receiving a signal. This is indicated by a double-head arrow 905 in FIG. 9. Thus, the time in which the power supply to the wireless LAN system can be turned off between the reception of a data packet and the reception of a next data packet decreases by the pull-in time. The pull-in time is indicated by the double-head arrow 905 and required for the PLL circuit to stabilize the frequency.
As the data transfer rate increases, the standby period decreases as noted above, but the pull-in time remains unchanged. The pull-in time is required for the PLL circuit to stabilize the frequency. In short, as the data transfer rate increases, the pull-in time has an increasingly greater negative impact on the reduction of the PLL circuit power consumption. The pull-in time is required for the PLL circuit to stabilize the frequency. To lower the power consumption of the PLL circuit, it is essential to cut the pull-in time short. The pull-in time is required for the PLL circuit to stabilize the frequency.
FIG. 5 is a block diagram of a common, conventional PLL circuit. A voltage-controlled oscillator 501 of the PLL circuit shown in the figure oscillates at a particular frequency fo according to an voltage value input at an oscillation control voltage input terminal 502, to output an oscillating signal from a voltage-controlled oscillator output terminal 503. The oscillating output is fed to an internal circuit of the system via the buffer amplifier 504 and directly fed to a prescaler 505. The prescaler 505 divides frequency to output a frequency-divided signal from an output terminal 506 to a phase comparator 507.
The phase comparator 507 is supplied with both the frequency-divided signal and a reference signal from a reference oscillator 508 constructed of, for example, a quartz oscillator. The phase comparator 507 drives a charge pump 509 with a phase difference signal on the basis of a phase difference between the two input signals, that is, the frequency-divided signal and the reference signal. Between the charge pump 509 and the voltage-controlled oscillator 501 is located a loop filter 510. The filter 510, constructed of a lowpass filter, smoothes, and removes noise from, the output signal of the charge pump for an output to the voltage-controlled oscillator 501.
The conventional PLL circuit includes the abovementioned loop through which the oscillating output of the voltage-controlled oscillator 501 is fed back to the voltage-controlled oscillator 501 as shown in FIG. 5. The feedback is repeated until there is no phase difference between the frequency-divided signal from the prescaler 505 and the reference signal from the reference oscillator 508. The frequency of the PLL circuit thus becomes stable. Ultimately, the PLL circuit oscillates at a predetermined frequency.
In conventional technology, the “in operation” value of the oscillation control voltage when the oscillation of the PLL circuit is stable is stored. Immediately after the circuit is turned on, a voltage setting means is used to force the oscillation control voltage to the stored value.
Referring to FIG. 6 to FIG. 8, the following will describe conventional art which is proposed to reduce the pull-in time for the PLL circuit. For convenience, the components of the PLL circuits shown in FIG. 6 to FIG. 8 that have the same arrangement and function as those in FIG. 5 are indicated by the same reference numerals and description thereof is omitted.
As a first conventional example, FIG. 6 is a block diagram illustrating the structure of the PLL circuit disclosed in Japanese Unexamined Patent Publication 1-305724/1989 (Tokukaihei 1-305724; published Dec. 11, 1989; hereinafter, “patent document 1”). As shown in the figure, the PLL circuit disclosed in patent document 1 has a read-only memory (ROM) 612 to reduce the pull-in time. The read-only memory 612 contains a table of oscillation control voltage values versus oscillation frequencies of a voltage-controlled oscillator 501 in the PLL circuit. A digital/analog converter (D/A converter) 613 produces a voltage required by the voltage-controlled oscillator 501 so that the oscillator 501 can oscillate at a predetermined frequency. A capacitor 511 in a loop filter 510 is charged in advance (precharged) while the PLL circuit is being powered off. This allows the voltage-controlled oscillator 501 to oscillate at a frequency close to a desired frequency when the power supply is turned on. It is thus possible to shorten the time the PLL loop takes to achieve a stable frequency.
As a second conventional example, FIG. 7 is a block diagram illustrating the structure of the PLL circuit disclosed in Japanese Unexamined Patent Publication 8-125527/1996 (Tokukaihei 8-125527; published on May 17, 1996) (hereinafter, “patent document 2”). As shown in the figure, the PLL circuit disclosed in patent document 2 includes a loop switch 712 between the charge pump 509 and the loop filter 510. The loop switch 712 is intended to hold the electric charge accumulated in the capacitor 511 in the loop filter 510 after the PLL circuit is turned off, so as to maintain across the capacitor 511 the control voltage being applied while the PLL circuit was in operation. In this manner, in the second conventional example, when the PLL circuit is off, the loop switch 712 maintains the control voltage being applied while the PLL circuit was in operation. This allows the voltage-controlled oscillator 501 to oscillate immediately at a frequency close to a desired frequency when the power supply is powered on. It is thus possible to shorten the time the PLL loop in the PLL circuit takes to achieve a stable frequency.
As a third conventional example, FIG. 8 is a block diagram illustrating the structure of the PLL circuit disclosed in Japanese Unexamined Patent Publication 2002-252561 (Tokukai 2002-252561; published on Sep. 6, 2002) (hereinafter, “patent document 3”). As shown in the figure, the PLL circuit disclosed in patent document 3, furthering the concepts of patent document 2, includes an automatic voltage control device 813 which records in an analog manner a voltage value of the loop filter 510 being applied while the PLL circuit is in operation. The input of the automatic voltage control device 813 is connected the loop filter output section 502 via the switch 814. The output of the automatic voltage control device 813 is connected to the voltage-controlled oscillator 501. When the PLL circuit is powered on again in an OFF state, the device 813 outputs the voltage value recorded in the automatic voltage control device 813 so that the voltage-controlled oscillator 501 can oscillate immediately at a frequency close to a desired frequency. The PLL circuit disclosed in patent document 3 thus shortens the time the PLL loop takes to achieve a stable frequency.
The structures of these first to third conventional examples have the following problems.
The first conventional example provides a potential to the loop filter when the power supply is turned on or a channel is altered. The capacitor in the loop filter needs to be precharged again every time the power supply is turned on/off. The structure may present an obstacle to reduction of power consumption, especially if it is employed in a system whose power supply is frequently turned on/off.
The second conventional example controls the loop switch between the charge pump and the loop filter so that the capacitor in the loop filter can hold charge while the PLL circuit is being off. The capacitor is not charged while the PLL circuit is being off. Therefore, the loop filter voltage may drop through leak of electric charge from, for example, a varactor diode connected to the control terminal of the voltage-controlled oscillator. Consequently, when the PLL circuit is turned on next time, the output voltage of the loop filter is lower than the control voltage being applied while in operation. This causes a shift in the initial frequency when the PLL circuit is turned on. The pull-in time shorting effects are thus reduced.
The third conventional example requires that a current supply circuit made primarily of a voltage generator circuit be separately provided. This may lead to increase in IC chip size and power consumption.